Digital communications are making more and more use of more or less sophisticated channel coding. The principle of channel coding is to add controlled redundancy to information bits to enable the receiver to detect and possibly correct transmission errors. The existing codes include in particular block codes which cause a code word of size nc to correspond to a block of nb information bits, with nc>nb. These additional bits are used to correct some transmission errors; the greater the number of redundant bits, the greater the correction capability of the code. To decode and correct errors, the receiver must be in a position to find the beginning of the code words in the bit sequence in order to be able to decode them correctly. This operation is usually referred to as frame synchronization.
The invention is concerned with frame synchronization techniques.
Frame synchronization is generally effected by periodically adding to the message to be transmitted an uncoded synchronization sequence known to the receiver. That sequence is then detected in the receiver to enable synchronization of the frame to be decoded. This kind of sequence is described in the following papers: “Optimum Frame Synchronisation”, James L. Massey, IEEE Transactions on Communications, Vol. Com-20, No. 2, April 1972, and “Frame Synchronisation Techniques” Robert A. Scholtz, 0090-6778/80/0800-1204$00.75, 1980 IEEE. The enormous advances in channel coding, in particular with the introduction of turbo codes, means that data can now, be sent with low signal-to-noise ratios, which leads to numerous errors in the synchronization sequence and therefore to somewhat unreliable synchronization. A number of authors propose inserting a coded sequence; this means that some of the improvement in coding can be exploited and synchronization reliability thereby increased. One such sequence is described in the paper “Decoder-Assisted Channel Estimation and Frame Synchronisation of Turbo Coded Systems”, M. Mostofa, K. Howlader, 0-7803-7484-3/02/$17.00, 2002 IEEE.
Those known techniques have the drawback of requiring the addition of a known sequence, which reduces spectral efficiency, i.e. lowers the usable bit rate.